MRI (Magnetic Resonance Imaging) and IMRI (Intraoperative Magnetic Resonance imaging) are well known diagnostic (MRI) and therapeutic (IMRI) tools in medicine. The remarkable soft tissue contrast resolution associated with these techniques is invaluable and renders these techniques high appreciation among the medical community.
Basically, the MRI technique is exploiting nuclear magnetism induced on the patient's tissues (a very clear explanation of the MRI principles is provided by Joseph P. Hornak, of the Rochester Institute of Technology, on the World Wide Web, http://www.cis.rit.edu/htbooks/mri/mri-main.htm, and see also U.S. Pat. No. 5,304,933). It is based on the fact that atoms with an odd number of protons or neutrons possess a weak but noticeable magnetic moment. Normally these magnetic moments are randomly oriented, but when subjected to a strong magnetic field (usually referred to as B.sub.0), they are forced to align. The static nuclear moment (spin) of the aligned nucleons under the strong magnetic field is too weak to be detected. Therefore, the aligned nucleon moments are tipped away from the z direction of the static strong magnetic field, using a weak rotating radio frequency (RF). The resonance frequency of a nucleon is called the Larmor Frequency. When perpendicular to the static field, the moment experiences torque proportional to the static magnetic field, which causes the spins to oscillate or precess in a plane perpendicular to the static field. As the precessing spins constitute a time varying flux, they produce a measurable current picked up in a loop antenna, arranged to receive the x and/or y and/or z components of the induced signal.
In order to distinguish between spins of identical atoms in different regions each of the regions of spin has to experience a unique magnetic field. Therefore a gradient in the magnetic field in applied, in the x, y (and even z direction, if a three dimensional image is to be constructed). The oscillating moments in the object to be imaged comprise an array of oscillators, which due to the gradient in the magnetic field have distinctive oscillation phase with respect to their spatial location.
The spins are subjected to a pulse of known properties which deflects the spins away from their magnetized orientation, and as they return to their original magnetized orientation they transmit a signal in the radio frequency which can be picked up and sampled by the antenna. The process is repeated n.times.m times, to produce an image with an n.times.m (or n.times.m.times.l, for a three-dimensional image, n, m and l being an integer) voxel resolution. The image is constructed using a known procedure involving the application of spatial Fourier Transforms performed on the signal received by the antenna, to produce a matrix of values representing gray-scale levels representation of the voxels.
MRI systems for performing whole body imaging employ large magnets that effectively surround the patient. These magnets are usually large superconductor magnets, taking up a large space (sometimes a room), are expensive and require high operating and maintenance costs. The large size of these magnets prevents any access to the patient.
However recently MRI systems for performing local imaging of specific body parts or organs were introduced. The basic concept of such systems is the realization that the soaring costs of whole-body imaging systems could be greatly reduced if smaller systems are constructed, taking also in consideration the fact that in most cases only a part of an organ of the patient's body needs imaging.
Israel Pat. Appl. No. 119558 (Katznelson et al.) filed Nov. 4, 1996, discloses a compact, transportable, intra-operative MRI System, which include a host computer coupled to a central electronics system which may be coupled to different MRI probes.
Compact MRI systems for performing local imaging of specific body parts or organs may use a hollow tube-like magnet assembly or other assemblies, such as two opposing magnets, such as described in U.S. Pat. No. 5,900,793 (Katznelson et al.), filed Jul. 23, 1997.
U.S. Pat. No. 5,735,278 (Hoult et al.), filed Mar. 15, 1996, disclosed an apparatus for use in surgical procedure comprising an operating table for receiving a patient for surgery and an MRI system for obtaining images of a part of the patient as a series of time through the surgical procedure for analysis by the surgical team to allow monitoring the progress of the surgery. The high field magnet and the operating table are shaped and arranged for positioning of the part of the patient into the magnetic field while the patient remains in place of the table and the magnet is mounted for movement between a first position spaced from the table and the patient thereon to allow the surgical team to carry out the surgical procedure and a second position for applying the magnetic field to the part of the patient. The table remains substantially stationary and only the magnet is moved to a position spaced from an adjacent end of the table to allow the surgical team to move around the adjacent end of table and to each side of the table to access the patient.
Usually an intraoperative MRI system (IMRI), such as the ones discussed above, would comprise an MRI system, with a magnet, positioned over an operating table. The magnet assembly is constructed so as to leave open spaces around the patient allowing the medical team to attend the patient. Another solution was the introduction of a magnet probe that can be brought near the patient lying on the operating table to perform the imaging, and then retracted to clear the way for the medical staff to access the patient. In Israeli Pat. Appl No. 119558 (Katznelson et al.) mentioned above, a transportable MRI system was introduced, that was intended to allow moving of the whole system within the operating room.
The cost of MRI systems--certainly the whole body imaging devices, but also smaller types of MRI systems--imposes a heavy toll on any hospital or other medical institute budget. Usually these medical institutions, which possess an MRI system, have only one such system, occupying a large spaced room, and serving as a designated imaging unit, with patients from various wards being sent to that unit to be imaged. Other institutions even prefer to cope without any MRI system, and rely on other institutions that have MRI systems to provide them MRI services.